Wharton professors, venture capitalists, and entrepreneurs are fueling an entire generation of Warby Parkers. Now there are more than 400 startups tackling products from toothbrushes to bras. What could go wrong?

As more startups execute the direct-to-consumer playbook with no real defensible moat (other than economics of scale), expect to see marketing costs rise to unsustainable levels, with a lot of these brands going out of business as a result.